This invention relates in general to temperature compensation in an apparatus and, more particularly, to a method and apparatus for effecting temperature compensation in an optical apparatus using a passive athermalization component to effect relative movement of different components.
An optical apparatus usually needs to operate reliably and accurately over a relatively wide temperature range. However, changes in temperature can affect optical characteristics of the apparatus. For example, where an optical apparatus has infrared-transmissive lenses made of a material such as germanium, the index of refraction of the lenses will change fairly significantly in response to a temperature change. Absent compensation, the change in the index of refraction can produce problems such as shifting of a focal plane relative to a sensor or some other component, thereby degrading the accuracy of the focusing function provided by the lenses.
In attempt to provide appropriate temperature compensation to avoid such problems, various techniques have previously been used. One such approach involves active focus compensation through use of temperature sensors, a closed loop control system responsive to the sensors, and motor-driven positioners that reposition the lenses under control of the control system. However, this type of approach is complex, and therefore cost prohibitive for many applications. Further, the complexity gives this system a level of reliability which is less than desirable.
Another known compensation approach effects passive repositioning of lenses through use of materials having different coefficients of thermal expansion. As the ambient temperature changes, the materials change dimensionally at different rates, in a manner which repositions the lenses so as to maintain a sharp focus. Prior systems of this type have included components such as fluid-filled bellows, bimetallic springs, and high expansion rate plastics. However, various additional parts are typically needed in order to address other considerations. For example, separate loading springs are provided to control backlash and maintain stiffness in vibration and shock environments. Further, separate parts are provided in order to achieve initial focus adjustment. In addition, these approaches often involve many parts that must be assembled, high-precision fabrication, and separate locking parts, compounds or processes. Therefore, as a whole, these approaches are fairly complex, and thus cost prohibitive for many applications.
A related consideration is that, in many prior approaches, setscrews are used to clamp and lock concentric assemblies, for example to maintain initial focus adjustment. However, the setscrews may tend to loosen over time. Further, the setscrews can deform threads on other structures, which can make it difficult or impossible to subsequently readjust the initial focus setting, and which can increase the binding between parts. Also, temperature changes may alter dimensional characteristics of various parts in a manner that effectively loosens the grip provided by the setscrews.
From the foregoing, it may be appreciated that a need has arisen for a better method and apparatus for effecting passive temperature compensation within an optical apparatus. According to the present invention, a method and apparatus are provided to address this need, and involve: supporting first and second parts for relative movement parallel to a first direction, the first part including a section having thereon a coupling structure which faces in a second direction approximately transverse to the first direction; providing on one of the first and second parts an optical component which is part of an optical system; and compensating for variation in optical characteristics of the optical system due to temperature changes by providing a compensation part which is operatively coupled to the second part and which includes a section having thereon a coupling structure that faces and operatively engages the coupling structure on the first part, a first of the sections having thereon a first of the coupling structures and a second of the sections having thereon a second of the coupling structures. The compensation part has a coefficient of thermal expansion which causes the compensation part to respond to a temperature change by undergoing a size change that effects relative movement of the first and second parts parallel to the first direction by an amount which compensates for the variation in optical characteristics of the optical system caused by the temperature change, and the first section continuously resiliently urges the first coupling structure against the second coupling structure in a direction approximately parallel to the second direction.